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3.0. EVALUATING ATMOSPHERIC RELEASES OF DIOXIN-LIKE COMPOUNDS FROM COMBUSTION SOURCES

3.1. INTRODUCTION

Since the late 1970's, it has become well established that the combustion of certain fuels containing both organic material and chlorides can form polychlorinated dibenzo-p-dioxins (CDDs) and poylchlorinated dibenzofurans (CDFs). This discovery has prompted world-wide research to identify combustion sources, to characterize the conditions favoring the formation of CDDs and CDFs within the combustion process, and to characterize the emission of dioxin-like compounds to the air from the stack of the process.

The purpose of this chapter is to provide site-specific procedures for evaluating the emission of dioxin-like compounds from stationary combustion sources. The first step is to characterize stack emissions in terms of mass of TCDD/F congener released, and then to partition that release into a vapor and a particle phase.

Using atmospheric transport modeling, these releases are translated to ambient air vapor and particle phase concentrations, and wet and dry particulate deposition amounts, in the vicinity of the release. This chapter demonstrates these procedures on a hypothetical incinerator using an air dispersion model called COMPDEP.

A second purpose of this chapter, therefore, is to provide the background and justification for the model inputs and key parameters for COMPDEP. The final results for this model simulation are vapor and particle phase concentrations, and particulate deposition amounts of the specific dioxin-like congeners, which are then used for the demonstration of the stack emission source category in Chapter 5 of this Volume.
This chapter is structured as follows:

. Section 3.2 describes the generation of congener specific emission factors. These factors are defined as the mass of congener emitted per mass of feed material combusted. Subsections within Section 3.2 discuss:

1) a heirachy of preferred options for generating such emission factors, starting with site-specific stack testing for specific congeners and ending with engineering evaluations when no other data is available,
2) an approach to estimating congener-specific emission factors if homologue group emission factors are all that is available, including a presentation of source category homologue group profiles generated with limited data,
3) the emission factors for the example incinerator demonstrated in Chapter 5, and assuming a feed rate into the example incinerator, emissions expressed on a mass per time basis (which is required for transport modeling),
4) partitioning of emissions into a vapor and a particle phase for atmospheric transport modeling, and
5) a procedure to estimate the mass released and concentrations for a related emission of a combustor, that of ash.

It is noted that a similar discussion on emission factors is presented in Chapter 3 on Sources in Volume II of this assessment. However, the Volume II discussion derives national average emission factors for various combustor types as a basis for estimating total annual releases to the air in the United States from each combustion source category in comparative units of grams of Toxic Equivalent (TEQs) emissions per year.

. Section 3.3
describes a general air modelling procedure for evaluating the fate and transport of dioxin-like compounds emitted from stacks. The discussion presents a general review of dispersion theory, a general review of dry particle deposition, and a general review of the wet deposition algorithm employed in this analysis. EPA's COMPDEP air dispersion and deposition model is reviewed. Wherever pertinent, Section 3.3 describes the assumptions and parameter values that were used in the demonstration of methodologies in Chapter 5 of this Volume.

. Sections 3.2. and 3.3.
summarized input data and assumptions (emission rates, vapor/particle partitioning assumptions, etc.) that were made for the demonstration of the methodologies for evaluating stack emissions in Chapter 5 of this Volume. Section 3.4 supplies all other key assumptions for the stack emission demonstration, such as stack height and exit temperatures, meteorological data, and others. This Section also provides the final results from the COMPDEP modeling, including vapor phase air concentrations at various distances in the predominant wind direction, and dry and wet deposition fluxes, also at various distances in the predominant wind direction.

. Section 3.5
closes out the chapter by summarizing critical aspects for making site specific evaluations of stack emission sources.

3.2. ESTIMATING THE EMISSIONS OF DIOXIN-LIKE COMPOUNDS FROM ANTHROPOGENIC COMBUSTION SOURCES

Estimating the emission factor is the first step in assessing a specific stack emission source of dioxin-like compound release. For this assessment, an "emission factor" is defined as the total mass (in vapor and particulate forms) of dioxin-like compound emitted per mass of feed material combusted.

This assessment recommends the generation of emission factors for individual dioxin-like congeners for a site-specific assessment. Chapter 3 on Sources in Volume II of this assessment also derives emission factors. However, the Volume II discussion derives national average emission factors for TEQ emissions for various combustor types as a basis for estimating total annual releases to the air in the United States in comparative units of grams TEQ per year.

In the past, EPA converted the concentrations of TCDD/F mixtures into an equivalent concentration of 2,3,7,8-TCDD (Cleverly, et al., 1989, 1991; EPA, 1987a; Mukerjee and Cleverly, 1987) when deriving an emission factor. The fate, transport, and transfer parameters of 2,3,7,8-TCDD were applied to model the environmental fate of this TEQ mixture. This perspective has been changed in the following procedure. In order to increase the level of accuracy, the air dispersion modeling was done separately for each of the toxic congeners. Only at the interface to human exposure, e.g., ingestion, inhalation, dermal absorption, etc., are the individual congeners recombined and converted into the toxic equivalence of 2,3,7,8-TCDD to be factored into the quantitative risk assessment. It is recommended that a site-specific assessment, or even an assessment of a source category, be based on congener-specific emissions rather than TEQ emissions.

Section 3.2.1 presents a strategy for development of emission factors for conducting a site-specific assessment. Section 3.2.2 describes an approach to estimating congener-specific emission factors when all that is available are homologue group emission factors. Section 3.2.3 summarizes the emission factors for the hypothetical incinerator demonstrated in Chapter 5. Section 3.2.4 presents an in-depth evaluation of the partitioning of emissions between a vapor and a particle phase for further atmospheric transport modeling. This discussion includes subsections on measurements of partitioning at the stack, measurements of partitioning in ambient air, and the theoretical approach used in this assessment for vapor/particle partitioning. Section 3.3.5 closes this section on emission factors by describing procedures to estimate the mass of ash (fly and bottom) produced and the concentration of dioxin-like compounds on ash.

3.2.1. A Strategy for Generating Emission Factors

The following is a heirarchal listing of data collection options for emission factors:

A. For facilities that are built and operational,
it is preferred that direct stack measurements be used, using EPA recommended congener-specific stack monitoring and analytical protocols. Stack monitoring provides concentrations and mass release rates of the pollutant, actual volume of stack gas and temperature. Care should be taken to ensure that the emissions characterization reflects a wide range of operating conditions and also accounts for deterioration in emissions output of the facility over its useful life. Procedures to convert data expressed in concentrations or mass release rates to an emission factor are as follows:

1. Test data of emissions are first placed into common units of measurement. English units are converted into metric, and the concentration term (mass of pollutant per unit volume of combustion gas emitted from the stack) should be corrected to the standard temperature and pressure on a dry gas basis, and standard percent carbon dioxide or oxygen within the combustion gas (e.g.,12% CO2). These adjustments may be necessary if more than one test occurred for stack emissions.

2. The next step involves converting the mass emission concentration of the specific dioxin-like congener in units of nanograms per normal cubic meter (at standard temperature and pressure) of combustion gas corrected to 12% carbon dioxide into an equivalent emission factor in units of grams of pollutant emitted from the stack per kilogram of combustable material or feed (g CDD and CDF/kg feed) that was incinerated at the facility during the duration of stack sampling.

This is done as follows:

Equation V3 3-1

3. As a final step, the average emission factor of each congener is derived by summing the emission factors and dividing by the number of data points used. The average should represent an approximation of long-term emissions. Many air dispersion models require emission factors in units of amount of the pollutant emitted per second of time. Therefore the average emission factor must be adjusted accordingly by adjusting the units in Equation (3-1) to a time-scale of one second.

 

B. For facilities that have been constructed, but not yet operational, or are in the planning stages of development,
the following procedure is recommended:

1. Collect and review stack test reports which have measured the emissions of specific dioxin-like congeners from facilities that are most similar in technology, design, operation, capacity, fuel, waste feed composition, and pollution control as the facility under consideration.

2. Determine if the stack test reports used EPA recommended stack monitoring and analytical protocols specific to dioxin-like compounds and discard those data not in conformance.

3. When combining data from a test results of a number of facilities, care should be taken to convert emissions, process feed rates and stack gas parameters to consistent units of measurement.

4. Ranges and average values should be developed for purposes of exposure analysis.

C. If no congener-specific data exists for a specific facility or similar facilities,
then use homologue profile emissions from similar facilities. Steps 1-4 in B above pertain here as well. Estimating congener-specific emission factors given homologue emission factors is described in Section 3.2.2 below.

D. If no data exist relevant to a specific facility, then the Compilation of Air Pollution Emission Factors
(EPA, 1985; and subsequent updates), should be used. This compilation was put together and is periodically updated by EPA's Office of Air Quality Planning and Standards (OAQPS), and is commonly referred to as AP-42. Care should be taken to select emission factors which were developed for technologies that best match the facility under consideration. The basic limitation of these of these data is the fact that emission factors are not usually reflective of specific emission control equipment.

OAQPS's AP-42 document provides TCDD/F emission factors for municipal waste combustors, sewage sludge incinerators, and medical waste incinerators. At this time, emissions from hazardous waste incinerators are not addressed in AP-42. Emission factors presented in AP-42 are designed for estimating emissions from a large number of sources over a wide area. They are averages of values determined at one or more individual facilities. The individual values which are used to develop the average may vary considerably. The use of AP-42 emission factors to estimate emissions from any one facility should be done with great care.

E. In the absence of suitable AP-42 emission factors,
clearly documented engineering evaluations may be used. Documentation should include copies of emission test reports used to derive the emission estimates, any assumptions made and the rationale for the conclusions that were made.

3.2.2. Use of Homologue Profiles for Estimating Congener Specific Emission Factors

This section describes emission factors for homologue groups of dioxin-like compounds from various stack emission sources. These emission factors are described in units of m g homologue emitted/kg feed material combusted. These are presented for comparative purposes only, and should not be interpreted as representative of the sources described. Most of the profiles are based on very limited data generated under limited emission controls.

When only homologue emission factors are available, then rough estimates of congener specific factors can be made. First, an equal probability of occurrence of the specific congener is assumed based on relative proportionality. For example, 2,3,7,8-TCDD is one congener out of 22 possible congeners in the TCDD homologue. Therefore, the probability of occurrence is assumed to be the ratio of 1/22 or 0.045. Multiplication of a total TCDD emission factor by 0.045 gives an approximation of the emission factor for 2,3,7,8-TCDD. Table 3-1 lists the number of dioxin-like congeners within a homologue group and the total number of congeners within that homologue group.

The use of this procedure may be the only way to evaluate source or site-specific emissions of dioxin-like polychlorinated biphenyls (PCBs), known as the coplanar PCBs. No congener specific test data of coplanar PCBs from incinerators or combustion sources could be found for this assessment. The data that are available usually have reported PCBs as the sum total of all PCBs present in the sample without further speciation of toxic congeners or congener groups (EPA, 1987b). The greatest level of detail in any test report is a further breakdown of total PCBs into homologue groups, e.g., mono - deca-chlorobiphenyl.

Figure 3-1 displays the homologue profiles for 11 specific source categories. Following now are brief summaries of the reference materials for these homologue profiles.

Again, it is emphasized that these homologue profiles are not offered as source generalities; they are mostly generated on a small number of different facilities not including a range of emission controls. It is not known whether tested facilities represent the average, or are higher or lower than typical facilities in each source category.

1. Municipal Solid Waste Incineration (MSWI):
Municipal incinerators can be classified into four general design categories: mass burn, modular, refuse-derive fuel (RDF), and fluidized-bed combustors (EPA, 1991). Figure 3-1 depicts the homologue profile of TCDD/Fs from MSWIs. It was constructed by merging emissions data from ten modern mass burn technologies (EPA, 1983; EPA, 1988a; Knisley, et al., 1986; Seelinger, et al., 1986; EPA, 1988b; Marklund, et al., 1985; Siebert, et al., 1991; Entropy, 1987).

table Table 3-1. The number of dioxin-like and total congeners within dioxin, furan, and coplanar PCB homologue groups.
2. Hazardous Waste Incineration:
Hazardous waste incinerators have not been extensively evaluated for stack emissions of dioxin-like compounds.

Only a few reports appear in the published literature from which homologue emission factors can be developed (NATO, 1988; EPA, 1992), and these do not give complete inventories of emissions.

Therefore, homologue emission factors were estimated based on a series of emission tests at a rotary kiln waste incinerator in Biebesheim, German (EPA, 1992).

Figure 3-1 depicts the homologue profile for this single hazardous waste incinerator.
expand table Table V3 3-1
 

3. Drum and Barrel Reclamation Furnace:
Dioxin-like compounds were measured by EPA in the stack gas emissions of a drum and barrel reclamation furnace as part of the National Dioxin Study of Combustion Sources conducted in 1986 (EPA, 1987d). The tested facility was judged by EPA to be typical of the industry. These plants operate a furnace to prepare used steel 55-gallon drums for cleaning to base metal. The cleaned drums are repaired, repainted, relined and sold for reuse. The used drums processed at the tested facility were from the petroleum and chemical industry.

table Figure 3-1 Homologue emission factors for source categories of dioxin-like compound releases.
expand table Figure 3-1A Municipal Waste Incinerators expand table Figure 3-1B Hazardous Waste Incinerators
Figure V3 3-1A Figure V3 3-1B
table Figure 3-1 Homologue emission factors for source categories of dioxin-like compound releases.
expand table Figure 3-1C Drum and Barrel Reclamation Furnaces expand table Figure 3-1D Hospital Waste Incinerators
Figure V3 3-1C Figure V3 3-1D
table Figure 3-1 Homologue emission factors for source categories of dioxin-like compound releases.
expand table Figure 3-1E Wire Rec expand table Figure 3-1F Tire Incinerators
Figure V3 3-1E Figure V3 3-1F
table Figure 3-1 Homologue emission factors for source categories of dioxin-like compound releases.
expand table Figure 3-1G Wood-Fired Boiler expand table Figure 3-1H Secondary Copper Smelts
Figure V3 3-1G Figure V3 3-1H
table Figure 3-1 Homologue emission factors for source categories of dioxin-like compound releases.
expand table Figure 3-1K Kraft Black Liquor Boilers expand table Figure 3-1L Sewage Sludge
Figure V3 3-1K Figure V3 3-1L
table Figure 3-1 Homologue emission factors for source categories of dioxin-like compound releases.
expand table Figure 3-1M Carbon Regeneration Furnaces
The drum burning process subjected the used drums to an elevated temperature in a tunnel furnace for a sufficient time so that the paint, interior linings, and previous contents were burned or disintegrated. The furnace was fired by auxiliary fuel. Used drums were loaded onto a conveyor that moved at a fixed feed rate. As the drums passed through the preheat and ignition zone of the furnace, additional contents of the drums drained into the furnace ash trough. A drag conveyor moved these sludges and ashes to a collection pit. The drums were air cooled as they exited the furnace. Exhaust gases from the burning furnace were drawn through a breaching fan to a high-temperature afterburner.
Figure V3 3-1M

The homologue profile for drum and barrel reclamation furnaces, shown in Figure 3-1, was developed from EPA stack tests of this operation (EPA, 1987d).

4. Medical Waste Incinerators:
The State of California Air Resources Board (CARB) has stack tested a number of hospital waste incinerators in southern California (CARB, 1990). Congener-specific emissions of PCDD/Fs were measured in the stack gas emissions of 7 facilities. Figure 3-1 displays the homologue profile constructed from the average emission of three facilities tested by CARB identified as facilities A - C in their summary overview of emissions (CARB, 1990).

5. Scrap Electric Wire Incineration:
Dioxin-like compounds emitted to the air from scrap electric wire incineration were measured from a facility during EPA's National Dioxin Study of combustion sources (EPA, 1987d). The objective of wire incineration is to remove the insulating material and reclaim the metal (e.g., copper, silver, and gold) comprising the electric wire, hence these facilities are sometimes referred to as wire reclamation incinerators. The reclaimed metal is then sold to a secondary metal smelter. The tested facility was judged by EPA to be typical of this industry. Insulated wire and other metal-bearing scrap material were fed to a combustion unit where incineration of the material was assisted by the combustion of natural gas. The estimated temperature during combustion was 650° C, and combustion transpired in both a primary and secondary chamber. The tested facility was equipped with a high temperature afterburner to further destroy organic compounds entrained in the combustion gases prior to discharge to the air from the stack. Figure 3-1 displays the homologue distribution developed from this single facility.

6. Automobile Tire Incineration:
Homologue emissions factors shown in Figure
3-1 were developed from an automobile tire incinerator stack tested by the State of California Air Resources Board (CARB, 1991). The facility consists of two excess air furnaces equipped with steam boilers to recovery the energy from the heat of combustion. Discarded whole tires are fed to the incineration units at a rate of 3000 kg/hr. The furnaces are equipped to burn natural gas as auxiliary fuel. The steam produced from the boilers is used to drive electrical turbine generators to produce 14.4 megawatts of electricity. The facility is equipped with a dry acid gas scrubber and fabric filter for the control of emissions prior to exiting the stack. These devices are capable of greater than 95% reduction and control of dioxin-like compounds prior to discharge from the stack.

7. Industrial Wood-Burning Facilities:
The homologue profile shown in Figure 3-1 for this source category was developed from measurements of stack emissions from an industrial wood-burning furnace (EPA, 1987d). The tested facility was judged by EPA as being typical of these combustion technologies. The facility was located at a lumber products plant that manufactures overlay panels and other lumber wood products. The wood-fired boiler tested was a three-cell dutch oven equipped with a waste heat boiler. During normal operation, the furnace is 100% fired with scrap wood from the lumber plant. The feed wood is typically a mixture of bar, hogged wood, and green and dry planar shavings. For the stack test from which the homologue profile was developed, the feed was mostly wood from fir and hemlock. Nearly all the wood fed to the lumber plant had been stored in sea water adjacent to the facility, and therefore had a significant concentration of inorganic chloride. The scrap wood fed to the boiler had not been treated with chemical preservatives, such as pentachlorophenol. The wood was fed to the boiler by a screw conveyor that dumps the feed into a pile in the primary combustion chamber. The furnace was operated at air in 50% excess of stoichiometric requirements. Boilers captured the heat of combustion and transfered the heat into steam for co-generation of energy at the plant. The exhaust gases from the boiler passed through a cyclone and fabric filter prior to discharge from the stack. The facility was equipped with a cyclone and fabric filter to control emissions. Emissions testing at this facility demonstrated that the fabric filter was reducing dioxin emissions by about 90% (EPA, 1987d).

8. Metal Reclamation Plants:
Metal reclamation plants are secondary metal smelting facilities which include secondary copper smelters, secondary aluminum smelters, secondary magnesium smelters, and secondary ferrous smelters. The only complete information with regard to the potential stack emission of dioxin-like compounds is from a secondary copper smelter tested by EPA during the National Dioxin Study (EPA, 1987d). The homologue profile shown in Figure 3-1 was developed from this facility. The tested facility was a secondary copper smelter that recovers copper and precious metals from copper and iron-bearing scrap. The copper and iron-bearing scrap was fed to a blast furnace, which produced a mixture of slag and black copper. The blast furnace was a batch-fed cupola furnace. Four to five tons of metal-bearing scrap were fed to the furnace per charge, with materials typically being charged 10 to 12 times per hour. Coke was used to fuel the furnace, which represented 14% (by wt) of the total feed. During the dioxin stack tests, the feed consisted of electronic telephone scrap and other plastic scrap, brass and copper shot, iron-bearing copper scrap, precious metals, copper bearing residues, refinery by-products, converter furnace slag, anode furnace slag, and metallic floor cleaning material. Oxygen enriched combustion air for combustion of the coke was blown up through the bottom of the furnace. At the top of the blast furnace were four natural gas-fired afterburners to aid in completing combustion of the exhaust gases. Particulate emissions were controlled by fabric filters, and the flue gas then was discharged into a common stack.

9. Kraft Black Liquor Recovery Boilers:
EPA stack tested three kraft black liquor recovery boilers for the emission of dioxin in conjunction with the National Dioxin Study (EPA, 1987d). The three sites were judged by EPA to be typical of Kraft black liquor recovery boilers, and the homologue profile shown in Figure 3-1 was derived from these three sites. These sources are associated with the production of pulp in the making of paper using the Kraft process. In this process, wood chips are cooked in large vertical vessels called digesters at elevated temperatures and pressures in an aqueous solution of sodium hydroxide and sodium sulfide (Someshwar and Pinkerton, 1992). Wood is broken down into two phases: a soluble phase containing primarily lignin, and an insoluble phase containing the pulp. The spent liquor (called black liquor) from the digester contains sodium sulfate and sodium sulfide that the industry finds beneficial in recovering for reuse in the Kraft process. In the recovery of black liquor chemicals, weak black liquor is first concentrated in multiple-effect evaporators to about 65% solids. The concentrated black liquor also contains 0.5% - 4% weight chlorides (EPA, 1987d). Recovery of beneficial chemicals is accomplished through combustion in a Kraft black liquor recovery furnace. The concentrated black liquor derived from the pulping process is sprayed into a furnace equipped with a heat recovery boiler. The bulk of the inorganic molten smelt that forms in the bottom of the furnace contains sodium carbonate and sodium sulfide in a ratio of about 3:1 (Someshwar and Pinkerton, 1992). The combustion gas is usually passed through an electrostatic precipitator that collects particulate matter prior to being vented out the stack. The particulate matter can be processed to further recover and recycle sodium sulfate.

10. Sewage Sludge Incineration:
EPA has conducted stack emission testing for dioxin from sewage sludge incineration at three multiple-hearth sewage sludge incinerators (EPA, 1987d). The homologue profile shown in Figure 3-1 was developed from tests on these three incinerators. Multiple hearth incinerators are the dominant technology in use in the United States today for the incineration of sewage sludge.

11. Granular activated carbon regeneration furnaces:
Granular activated carbon (GAC) is an adsorbent that is widely used in the control of pollutants in wastewater discharged from chemical and pharmaceutical industries, and in the treatment of finished drinking water at water treatment plants. Industrial manufacture of activated carbon is mostly obtained from the heat treatment of nut shells and coal under pyrolytic conditions (Buonicore, 1992). The properties of GAC make it ideal for adsorbing and controlling vaporous organic and inorganic chemicals entrained in combustion plasmas, as well as soluble organic contaminants in industrial effluents and drinking water. The high ratio of surface area to particle weight (e.g, 600 - 1600 m2/g), combined with the extremely small pore diameter of the particles (e.g., 15-25 Å) increases the adsorption characteristics (Buonicore, 1992). GAC will eventually become saturated and the adsorption properties will significantly degrade. When this occurs, the GAC usually must be replaced and discarded, which significantly increases the costs of pollution control.

The introduction of carbon reactivation furnace technology in the mid 1980's created a method involving the thermal treatment of used GAC to thermolytically desorb the synthetic compounds and restore the adsorption properties for reuse (Lykins, et al., 1987).The used GAC can contain compounds that are precursors to the formation of PCDD/Fs during the thermal treatment process. EPA measured precursor compounds in spent GAC used as a feed material to a carbon reactivation furnace tested during the National Dioxin Study (EPA, 1987d). The total chlorobenzene content of the GAC ranged from 150 ppb to 6,630 ppb. Trichlorobenzene was the most prevalent species present, with smaller quantities of di- and tetra-chorobenzenes detected. Total halogenated organics were measured to be about 150 ppm. EPA has stack tested two GAC reactivation furnaces for the emission of dioxin (EPA, 1987d; Lykins, et al., 1987). The homologue profile shown in Figure 3-1 was developed from the tests at these two facilities.

One facility was an industrial carbon reactivation plant, and the second facility was used to restore GAC at a municipal drinking water plant. The industrial carbon regeneration plant processed 36,000 kg/day of spent GAC used in the treatment of industrial wastewater effluents. Spent carbon was reactivated in a multiple-hearth furnace, cooled in a water quench, and stored and shipped back to primary chemical manufacturing facilities for reuse. The furnace fired natural gas, and consisted of seven hearths arranged vertically in series. The hearth temperatures ranged from 480° C to 1000° C. The spent GAC contained about 40% weight moisture. The used GAC was fed to the top hearth. In the furnace, the spent carbon was dried and the organics adsorbed onto the carbon particles were volatilized and burned in the heated combustion atmosphere.

The regenerated carbon dropped from the bottom hearth of the furnace to a quench tank to reduce the temperature. Air pollutant emissions were controlled by an afterburner, a sodium spray cooler, and a fabric filter. Temperatures in the afterburner were about 930° C. The second GAC reactivation facility tested by EPA consisted of a fluidized-bed furnace located at a municipal drinking water treatment plant (Lykins, et al., 1987). The furnace was divided into three sections: a combustion chamber, a reactivation section and a dryer section. The combustion section was fired by natural gas, and consisted of a stoichiometrically balanced stream of fuel and oxygen. These expanding gases of combustion provided heat, and suspended and fluidized the carbon. Temperatures of combustion were about 1,000° C.

The reactivation section outside the combustion chamber allowed for the complete volatilization of the heated GAC. Off-gasses from the reactivation/combustion section were directed through an acid gas scrubber and high-temperature afterburner prior to discharge from a stack. Another combustion process for which emissions data were sought was coal combustion in electric power generating facilities (utility boilers). Currently there is conflicting and extremely limited data on emission of dioxin-like compounds from coal-fired utility boilers (NATO,1988). The few published results of stack testing and monitoring of emissions from facilities in the United States have shown that dioxin has not been detected in stack gas emissions (NATO, 1988). Therefore, a homologue profile for this source category was not developed. The federal Clean Air Act requires an assessment of stack emissions of toxic air contaminants, including CDDs and CDFs, from coal-fired utility boilers. The EPA is currently collaborating with the U.S. Department of Energy in stack sampling seven facilities for CDD and CDF emissions. These results will be included in the final version of this document.

A homologue profile also could not be developed for a industrial category of potential concern, portland cement kilns. The database on stack emissions of dioxin from these units is just now becoming available. Volume 2, Chapter 3 of this assessment reviews current emissions inventories for cement kilns burning and not burning hazardous waste as auxiliary fuel in the production of cement clinker. In evaluating the TEQ of the mixture of CDDs and CDFs discharged from the stack of individual facilities, it became apparent that there was no consistent pattern to the relationship of total CDDs and CDFs to the estimated dioxin TEQ. For example, the ratio of total PCDD/Fs to the TEQ ranged from about a factor of 5:1 to a factor of 1000:1. A lower ratio reflects a skewing towards penta and tetra-chlorinated congeners in the distribution, and a higher ratio reflects a greater proportion of hexa, hepta, and octa chlorinated congeners in the emissions. Until more information becomes available from stack testing additional sources, a homologue profile of this industry will not be derived from the existing data.

3.2.3. Estimation of Emissions of Dioxin-Like Compounds from the Hypothetical Incinerator

The emission factors for the dioxin-like compounds from the stack of the hypothetical waste incinerator were derived from actual stack monitoring and emissions testing of an incinerator burning a complex mixture of organic waste. The concentrations of the specific PCDD/F congeners in units of nanograms per dry standard cubic meter (at 20° C; 1 atm.; 7% O2) were available, as were the volume of gas escaping from the stack and feed rates for the material being combusted during the stack tests.

Using procedures described in Section 3.2.1, this data was converted to emission factors. Such factors for three test runs are shown in Table 3-2. The fourth column is the average of these emission factors converted to g/sec units, which are the appropriate units for the application of the COMPDEP model. The conversion assumed a constant feed rate of 200 metric tons of feed material per day (further details on the hypothetical incinerator are found in Section 3.5). Human exposures to the coplanar PCBs emitted from a combustion source is not demonstrated in Chapter 5.

Therefore, an estimation of congener-specific emission factors of coplanar PCBs for the hypothetical incinerator are not provided. In order to put the emissions from the hypothetical waste incinerator into perspective, they can be compared with emissions from other incineration sources that are similarly controlled, e.g., equipped with scrubbers and/or fabric filters.Such air pollution control devices can reduce the amount of dioxin that is formed within the system by >99% prior to the release from the stack.

In this comparison, emissions from the following types of incineration processes were used (CARB, 1990; EPA, 1993): medical waste incineration; hazardous waste incineration; sewage sludge incineration; and municipal solid waste incineration. For comparisons, all emissions factors are expressed in units of nanograms TCDD-TEQ (Toxic Equivalent) emitted from the stack per kg of waste combusted, and are presented as ranges in measurements (minimum to maximum). This should not be confused as typical of the incineration source category, but specific only to sources having scrubbers and/or fabric filters.

Volume 2, Chapter 3 of this assessment gives an overview of dioxin emissions from incineration technologies equipped with a variety of pollution control systems. The emissions from the hypothetical incinerator is ranked with the other types of waste incinerators that are well controlled with some combination of a scrubber device and/or a fabric filter, as follows:

1. Medical waste incineration: 25 - 200 ng TEQ/kg waste combusted.

2. Hazardous waste incineration: 0.18 - 119 ng TEQ/kg waste combusted.

3. Hypothetical waste incinerator: 4.5 ng TEQ/kg waste combusted.

4. Municipal solid waste incineration: 0.05 - 3 ng TEQ/kg waste combusted.

5. Sewage sludge incineration: 0.002 - 0.03 ng TEQ/kg sludge combusted.

From these comparisons it appears that the TCDD-TEQ emission factor derived for the hypothetical incinerator lies well within the range of emission factors developed from measured incineration sources burning a diversity of waste material, but employing similar air pollution control technology. The hypothetical incinerator was arbitrarily assigned a waste combustion rate of 200,000 kg waste/day. This charging rate conforms to a large medical waste incinerator, an average hazardous waste facility, and moderate sewage sludge and municipal waste incinerators.

3.2.4. Estimation of the Vapor Phase/Particle Phase Partitioning of Emissions of Dioxin-Like Compounds

table Table 3-2. Emission factors and average emissions used for the hypothetical incinerator.
The first step in the air modeling is the partitioning of total emissions into a vapor and a particle state.

This section will review data on partitioning at the point of stack emission, in ambient air, and a theoretical approach to estimating the partitioning of dioxin-
like compounds in ambient air.

The true vapor/particle partitioning of dioxin under different conditions has not been directly measured, and therefore, is usually implied from these limited data or by theoretical means.
expand table Table V3 3-2